1. Field of the Invention
The present invention relates to controlled application of a force, and in a specific arrangement concerns an ultrasonic wire bonder in which there is minimized variation of pressure applied by the bonding tool tip to the object being bonded.
2. Description of Related Art
In manipulation of objects and performance of different types of processes, contact between a tool or object handler and an object being handled or worked upon, is often preferred to exert a known and controllable force upon the object. For example, in an ultrasonic wire bonding machine a slender, elongated tool is ultrasonically vibrated and moved against a wire to be bonded with a predetermined pressure while the tool tip is vibrated, in a direction parallel to the wire end under the tool tip, to create a bond between the wire and a bonding pad upon which the wire is placed. Characteristics of the bond are determined by a number of parameters of the bonding operation. One of such parameters is the amount of pressure or force applied by the bonding tool against the wire to press the wire against the bonding pad during application of ultrasonic bonding energy. If such bonding pressure should vary from one bond to another, or if it should vary during the performance of a single bonding operation, the resulting bond may be less desirable and less reliable, and has a greater probability of failure.
In performing an ultrasonic wire bonding operation, the tool tip is moved relative to the workpiece to a point over the area to be bonded, a length of wire is fed along the vertical tool to position an end of the wire beneath the tool tip. The tool tip often has a groove to locate and orient the wire. The tool and wire, which is in the tip groove, are then moved vertically downwardly until the wire contacts the workpiece or bonding pad to which it is to be bonded and is pressed against the pad by pressure of the bonding tool. The tool is then ultrasonically energized to form the bond while pressure continues to be applied to the wire by the tool. Having completed a bond, the tool is raised vertically and then moved to an area for making a subsequent bond. Great care must be taken in controlling the nature of the initial contact of wire and bond pad and the magnitude of force applied during application of ultrasonic energy.
In the past, force applied by the bonding tool has been controlled by a compliant mounting of the tool to the bonding carriage. U.S. Pat. No. 4,598,853 of William H. Hill for Open Center Flexural Pivot Wire Bonding Head, which is assigned to the assignee of the present application, describes a flexural pivot structure that pivotally mounts the bonding tool to its carriage upon a pair of leaf springs. The spring mount permits a relatively large amount of vertical motion with a minimized tip skid. Tip skid is the term used to describe the small amount of horizontal motion that takes place upon initial contact of the effectively pivotally mounted bonding tool. The springs help to provide the desired amount of vertical or Z axis preloading, e.g. the desired pressure of tool tip against wire during application of ultrasonic energy.
Another type of open center spring loaded pivot mounting for a wire bonding tool is described in U.S. Pat. No. 4,718,591 of William H. Hill, for a Wire Bonder With Open Center of Motion, which is assigned to the assignee of the present application. In U.S. Pat. No. 4,718,591 of Hill, as in his U.S. Pat. No. 4,598,853, magnitude of a force applied by the bonding tool to the wire during the ultrasonic bonding is determined by the springs which resist the pivotal deflection of the tool after it initially contacts the workpiece. The vertically movable tool carriage is provided with a small amount of over travel (travel beyond the point of initial touchdown of the tool tip) to provide a selected amount of preloading. Thus in the prior operations, the tool tip pressure is determined by the deflection of the springs which mount the tool to the bonding carriage and also by the carriage position. Amount of spring deflection is in turn determined by the amount of over travel. However, application of a constant force by the described arrangements may be hindered by one or more of the following factors: mechanical vibrations in the equipment, occurrence of wire deformation during application of ultrasonic energy, variability in the Z axis (vertical) position of the top of the bonding pad, and, in addition, touch sensitivity of the bonding tool. All of these factors tend to move the tool vertically to change spring deflection during application of ultrasonic energy.
Touch sensitivity is the ability of the tool mechanics and logic to determine the precise point of touchdown (the point from which amount of over travel is measured). Touchdown is generally signaled by a pair of switch contacts which are separated at the occurrence of touchdown. Precision of the point of signaling is adversely affected by variation in the relative motion of the contact elements, the finite incremental angular accuracy of the motor position encoder, and the time required for system logic circuits to sense and signal this occurrence. Bonding pads may be of different thicknesses, or may be secured to the substrate with different amounts of adhesive, and thus the height of the pad top surface above the substrate may vary.
During the bonding operation the wire, which is initially of circular cross section, is deformed under pressure to a considerably flatter cross section and thus elevation of the bonding tool will change during the actual bonding operation.
Still further, mechanical vibration of the equipment, generally in the order of ten to fifteen cycles per second, will cause variation in magnitude of force applied during the bonding operation. Particularly in an automatic bonding operation, where it is desired to perform bonding operations rapidly, the equipment is moved from one position to another at high speed and stopped at the desired position to perform the bonding operation. Because of the desire for high speed operation, the equipment may not be completely in its static condition for a time sufficient to allow vibrations occurring during the dynamic motion to subside before initiation of the bonding. Accordingly, such vibrations may still exist during at least a portion of the bonding operation.
All of these factors, touch sensitivity of the tool, wire deformation, bonding pad placement height and mechanical vibration, individually and/or collectively contribute to variation in the magnitude of a desired force to be applied by the bonding tool, and thus tend to result in bonds that are of less than optimum reliability.
Accordingly, it is an object of the present invention to provide a method and apparatus for applying force to an object while avoiding or minimizing above-mentioned problems.